If you select "Join Custom Game" and search for a map - for example Xel'Naga Caverns - you can join the game and play against a seemingly random player who had created the open-to-public custom game.

But how does battle.net select the game? Is MMR considered? Is it purely random? First come, first serve?

Similarly, is it possible to spectate open-to-public games on the standard maps? I'm thinking this could be an interesting alternative to watching live streams.

Edit: As answered by @Resorath, it seems to be first-come-first-serve. Do people find this feature useful / fun then? I would think it's pointless to play against opponents that have no relation to your skill level. I guess it could be interesting for practicing a risky build order without damaging your ladder ranking, if you don't have a practice partner.

Like you said, it's good for testing out builds. It's also a great way to get exposure to a new map before going on the ladder with it. For example, at the beginning of season 2 when they added in a lot of new maps to the ladder pool. This would allow you to test maps like Backwater Gulch without affecting your rating. With season 3 coming out soon you'll find more use as well since, again, they are adding new maps to the ladder pool.
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SoreanJul 4 '11 at 19:14